What do you get New York’s mayor, who has everything, to celebrate his 70th birthday? What else — burned toast. A dozen senior staffers surprised Mayor Bloomberg yesterday with the unusual charred offering, studded with birthday candles, at the Viand coffee shop near his Upper East Side townhouse. We’re told Bloomberg’s curious culinary tastes run to burned toast for breakfast, topped with a generous sprinkling of salt. To get him to the Viand, his team booked a fake 9 a.m. meeting with Mike Fishman, head of building workers’ union Local 32BJ. The Valentine’s Day birthday boy was also spotted having lunch at Circo with girlfriend Diana Taylor and six others. After he dined on flambéed shrimp, Taylor was heard whispering to staff, “Put a candle in a piece of cake . . . It’s his birthday.” Bloomberg was later sent a giant mousse and ricotta cheesecake from Staten Island civic activist Dr. Mohammed Khalid. To close out his 70th, he had a private dinner with friends and family.